The present invention relates to state variable sine wave oscillators having leveling circuitry for sensing the output amplitude and modulating the gain of a feedback path to stabilize and control the output amplitude.
A commonly known oscillator topology is the state variable oscillator in which the oscillator circuit provides an analog solution to a second order differential equation resulting in a sinusoidal output signal. The differential equation takes the following form: ##EQU1##
The instantaneous amplitude of the oscillator signal is represented by the term V; and the constants a, b, and c establish the frequency of the oscillator signal. Such circuits typically employ an inverter amplifier stage followed by first and second consecutive integrating amplifier stages, the output from the first integrator being fed back both to inverting and noninverting inputs of the inverter amplifier stage, and the output from the second integrator being fed back to the inverting input of the inverter amplifier stage. Each of the stages comprises an operational amplifier having associated corresponding input and feedback impedances which may be varied to produce oscillation at different frequencies and amplitudes.
All oscillators require leveler circuitry to stabilize and control their output amplitudes. Most leveler designs sense the oscillator's output amplitude, convert the amplitude into a DC control signal (with some inevitable AC ripple), and modulate the gain of a feedback path as necessary to maintain the desired output amplitude. Unfortunately the AC ripple in the control signal causes harmonic distortion of the oscillator output by the mixing of the fundamental oscillation feedback signal with the undesired AC ripple component of the control signal.
As mentioned in Hofer, A Comparison Of Low Frequency RC Oscillator Topologies (paper presented at 64th AES convention), the state variable oscillator topology is inherently superior to other types in its rejection of such leveler-induced distortion; however, further improvements in rejection are desirable.
Certain approaches for improvements in such rejection have been suggested by Meyer-Ebrecht, Fast Amplitude Control Of A Harmonic Oscillator, Proc. IEEE (Lett.), volume 60, page 736, June 1972; Vannerson and Smith, Fast Amplitude Stabilization Of An RC Oscillator, IEEE J. Solid State Circuits, volume SC-9, pages 176-179, August 1974; and Vannerson and Smith, A Low Distortion Oscillator With Fast Amplitude Stabilization, Int. J. Electronics, volume 39, pages 465-472, 1975. These approaches employ sample/hold or multiple phase/detection circuits to reduce the distortion. Another possibility is to use switchable filters or leveler range-changing circuits to reduce the ripple component of the leveler control signal. However, none of these suggestions is free of drawbacks and therefore none offers an optimum solution to the problem.